Apparatuses of the type outlined in the foregoing are already known. Thus U.S. Pat. No. 3,295,512 (Thomas J. McCusker) describes an apparatus which comprises a single conical reflector inside which a column of parabolic back-reflectors is arranged. The assembly of this apparatus is such that each incident solar ray is reflected first inwards by the conical reflector, then outwards by one of the parabolic back-reflectors and again inwards by the conical reflector, which focuses the solar rays, which have thus undergone triple reflection, on to heat collecting means whose centre coincides with that of the base (or input surface) of the conical reflector.
One of the drawbacks of this known apparatus is that its parabolic back-reflectors are difficult to construct with a satisfactory geometrical shape.
Another drawback is that the heat-collecting means are situated at a position which is difficult to reach, and constitute an impassible obstacle for the incident solar rays. The space available is therefore insufficient to house, inside the conical reflector, the heat-collecting means and their connections with the outside of the apparatus, if it is desired to limit to a reasonable value the central fraction of the conical reflector which is rendered unusable by the presence of these collecting means.
A third drawback resides in the fact that the solar rays all undergo triple reflection which considerably reduces the energy efficiency of the apparatus.